The sensitivity, signal quality, and the axial spatial resolution of current bio-imaging and probing systems using Optical Coherence Tomography (OCT) technology are primarily limited by the power and the bandwidth of the broadband light source. With recent advances in solid state and various optically and electrically driven light emitter technologies, there are no substantial shortages for light sources in the visible, near-mid-far infrared spectra. However, the development of a Gaussian spectrum and spectrally flat optical output light source that spans across a wide spectrum range remains a great challenge.
Generally, broadband light sources used in the current OCT system are obtained from four main sources: incandescent/halogen light sources, optically pumped crystal lasers, such as Ar-ion pumped Ti:Al2O3 laser, optically pumped fiber based amplified spontaneous emission (ASE) source, and semiconductor superluminescent diodes (SLDs).
Although incandescent/halogen light sources have broad bandwidths, they do not have sufficient intensity in a single spatial mode for high-speed imaging. Furthermore, the source exhibits major drawbacks, such as instability, poor reliability, and low energy efficiency. Typically, optically pumped crystal lasers utilizes non-linear optical or optical parametric effects to generate a broad band light. Although ultra short pulse laser sources have very broad spectral bandwidths (up to 350 nm) and high single-mode power, however the spectrum profile is not perfectly Gaussian and is limited to the emission in the near-infrared region (centered at ˜800 nm).
Optically pumped fiber lasers produce ultra broad non-Gaussian spectral bandwidths, but such operation requires a high power laser for optical pumping and an optical filter to produce a Gaussian spectrum. Moreover, the majority of these sources are expensive, bulky as most of them are table top systems, and consume very high power as the energy efficiency is low, thus presenting a major challenge to the widespread adoption of ultra high resolution OCT imaging technology in the clinical setting.
Semiconductor SLDs are particularly attractive for many imaging and sensor system applications due to their compactness and low energy requirement. The axial resolutions from available SLDs are typically limited to 10-15 μm for a device with a bandwidth of 20-30 nm at the center wavelength of 800 nm. Although not economic, techniques such as the combination of two or more SLDs with dissimilar wavelengths have been used to broaden the spectrum width, hence improving the axial spatial resolution of the OCT system to about 1 μm in scattering tissue and the retina. Different approaches have been used to further decrease the coherence length by broadening the spectral width of the SLDs.
Quantum-wells with different thicknesses and/or compositions in an active region have been used to expand luminescence spectrums. Due to the combination effects of different quantum-well carrier capture rates, low luminescence efficiency caused by the inter-well tunneling and photon re-absorption, SLDs fabricated using this technique usually give non-spectrally-flat optical output and low emission efficiency. In addition, the spectrum width is limited to the number of asymmetric quantum-wells that are used.
Broad luminescence width can be achieved by changing the quantum-well bandgap energy using selective area epitaxy or shadow masked growth. These techniques require extremely complex process optimization and design. The spectrum width of the SLDs using these processes is very much limited to 85 nm for most semiconductor structures.
Prior art lattice interdiffusion, or intermixing, or disordering processes are based on the fact that a quantum well (QW) system is inherently a meta-stable system due to the large concentration gradient of atomic species across the QWs and barriers interface. The process involves the introduction of beneficial defects to the material. During thermal annealing, the introduced impurities or created point defects alter the Fermi level and the high temperature enhances the solubility of certain point defects, which result in an increase in atomic interdiffusion rate and promote intermixing. It results in an increase of the bandgap energy when the energy profile changes from abrupt to graded QW bandgap profiles.
Although intermixing techniques have shown significant advantages over methods involving several growth and regrowth steps or spatially selective epitaxial growth for the bandgap engineering process, the spatial control of the degree of bandgap energy shift across a wafer using the existing quantum-well intermixing (QWI) techniques is indirect and complicated. For example, the spatial control of the bandgap across a GaAs/AlGaAs QW structure based on the sequential use of SiO2 masks with different thicknesses as QWI sources for impurity free vacancy diffusion (IFVD) or as implant mask for impurity induced disordering (IID) usually leads to a multiplication of the number of lithography, etching and dielectric cap deposition steps.
To address this issue, a one-step spatially controlled QWI technique based on IFVD was previously proposed. In this process, the semiconductor is patterned with very small areas of SrF2 followed by coating the sample with SiO2. The SrF2 patterns act as the bandgap control mask, while the SiO2 layer acts as an intermixing source. The degree of intermixing is then found to depend on the area of semiconductor surface in direct contact with the SiO2 layer. Although this technique is a one-step process, it requires complex process design including e-beam lithography and lift-off process to obtain the desired features for the implant mask.
Using the IID technique, selective area intermixing across a wafer can also be obtained with variable thickness SiO2 implant mask. However, this technique involves multiple lithography and etching steps. This can add significant process complexity, therefore illustrating the need for development of a simpler, single-mask processes to obtain the multiple bandgap QWI. In addition, this process needs to be performed at high implantation energy of ˜1 MeV, which results in the formation of extended defects such as vacancy clusters in the material and makes the complete recovery of the material damage a challenging task.
Based on similar process principles to the variable thickness of SiO2 implant mask, a significantly improved technique to achieve multiple bandgaps or graded bandgap across an InGaAs-InGaAsP laser structure was developed using a one process step technique. This process utilized grey scale mask lithography and near surface low-energy IID techniques to control surface point defect density, which in turn, spatially control the bandgap across an InGaAs/InGaAsP laser chip. In brief, spatial control of a SiO2 mask thickness is achieved by using a grey mask lithography technique followed by a one-step reactive-ion etching (RIE) process.
Although the grey mask low-energy IID is a simple process, and requires only a one-step lithography and intermixing step to create multiple bandgaps across a QW wafer, this technique is an expensive approach and requires a precise control of the SiO2 implant mask thickness. Furthermore, the critical process is to obtain a precise iso etch rate pattern transfer capability with precise control using RIE process.
Versatile intermixing has to be complemented with a robust material system to potentially offer large bandgap tunability. Due to the finite band offset of the material, most of the intermixing processes developed in material systems such as GaAs/AlGaAs and InGaAs/InGaAsP produce relatively narrow wavelength tunability. In an InGaAs/InGaAsP laser structures for example, a differential bandgap shift of only up to about 100 meV between the intermixed and non-intermixed regions can typically be obtained.
Most of the existing prior art post-growth multiple-bandgap engineering technologies require complex sample preparation and process steps. Also, the bandgap tuning range of these processes is relatively narrow.